Cheap (Free?) Mirror Replacement For Your CO2 Laser

hdd mirrors

You know what’s expensive? Those little tiny little mirrors used in laser cutters — and they don’t last forever either! What if we told you it’s possible to make your own for free, using a broken hard drive?

[Tim Wehr] read about using HDD platters as mirrors on, and decided to try it out for himself to see how well they work. He quickly salvaged an old hard drive and removed the ever so shiny platters. Using a few pieces of wood he clamped the platter and then cut circles out of it using a metal hole saw — the edges are a bit rough, so we’d recommend you invest in a diamond hole saw if you’re planning on trying this.

Some denatured alcohol polishing later and a bit of filing on the edges, and he had a replacement mirror. He then performed two tests using both the original and the HDD mirror on his CO2 laser. Almost identical cutting power. In fact, [Tim] muses that the HDD mirror looks like it cut slightly better even! Not bad!

[Thanks Riva!]

29 thoughts on “Cheap (Free?) Mirror Replacement For Your CO2 Laser

  1. why are the mirrors so much?

    it isnt like they are made of or coated with some kind of rare or restricted substance that requires a license or even haz mat license to handle and buy?

    1. The quality of the coating. A front-silvered mirror used in a lab setting usually has a surface flatness of lambda/10 (to preserve the quality of the wavefront) and a scratch-dig specification of 25-10 (to prevent scatter). So assuming a 10 micron wavelength for a CO2 laser, the variation between the high and low points on the surface of the mirror has to be less than 1 micron, and any scratches or scuffs on the surface must be less than 25 and 10 nm respectively.

      That’s just the glass that the mirror coat is deposited onto. Getting an acceptable, uniform coating as described above requires some serious hardware.

      1. “Getting an acceptable, uniform coating as described above requires some serious hardware.”

        Yep. Usually a very large, walk-in high-vacuum chamber for sputter-coating as they’re often coated in batches. *really* awesome stuff if you ever have the chance to see it in person ;)

        1. And then don’t forget all the special packaging and handling required to ensure that those high tolerances are maintained from the point of finishing the glass’ surface to the point it’s delivered to the customer. Also, don’t forget to take into account the fact that lab grade optics are being produced for a vastly smaller market than hard drive platters. The significantly smaller economies of scale also probably play a big part in the price difference.

    1. Easy test: Accelerate (throw) drive towards (hard) floor then pick up after it stops bouncing. If the contents of the drive rattles, it is (was) ceramic. I don’t think I’ve ever encountered a dead drive of the 3.5″ variety that had ceramic platters however.

      1. It’s still made of glass/ceramic at that point, you’ve just multiplied the number of platter in the drive. If you can manage to throw a hard drive at the floor hard enough to convert the platters from glass/ceramic to another kind of matter then you have a very promising career waiting for you as a Major League Baseball pitcher…

        1. I only bring this up because I worked with an older tech who used to “make sure” a hard drive was dead by literally slamming it off the floor several times. We always enjoyed the 2.5″ drives because you could hear them shatter.

          Surprisingly, it doesn’t take that much force for the ceramic platters.

  2. Interestingly, I just read up on this process over the last few days as I have a ‘new’ eBay laser with a cracked mirror. I didn’t have a drill press handy, so instead I hand shaped a piece of copper PCB, polished it with 600, 800, 1000, 2000 sandpaper and then metal polish. It’s been working great for a week now, though it may not have the longevity of hard drive mirrors or silicon/gold.

  3. To prevent the edges from coming up, clamp the platter between something hard(like metal) and put your clamps as close to the hole you are going to drill as possible to make sure the metal pushes onto the platter. Also, put the good side of the platter up.

  4. I have my laser on a large unit at the bottom of my bet and I can unclip a vent cover off and put the exhaust through it to outside. I made the mirrors from hard drive but after fitting them and firing a test shot at the mirror it went right past me and lit my bedding so take care the angles can be along way from original settings. Luckily I turned round and saw it and was able to put it out.

  5. I fastened a piece of wood 1″x1″ under a piece off wood that the hdd disk would sit on then i put 3 screws with washers on at 3rds of the way round the disk pulled up tight and used a 25mm starett hole cutter with no drill in the mandrill and started slow then as soon as there is a ridge it was easy to put more pressure on the drill and I cut 4 disks in about 2 mins then touched the edges on my fine bench grinder.

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